1. Field of the Invention
The present invention relates to thin film deposition, and is particularly related to physical vapor deposition (PVD) of solid materials in connection with the fabrication of semiconductor integrated circuits. More particularly, this invention is related to a method for controlling the target erosion and process characteristics in a physical vapor deposition sputtering source. This invention relates to any sputtering source utilizing sputtering targets. However, specific teachings and examples will be given which relate to magnetron sputtering, and especially to hollow cathode magnetron (HCM) sputtering sources.
2. Description of Related Art
In a sputtering device, a target is subjected to bombardment by high energy ions to dislodge and eject material from the target onto a workpiece, such as a semiconductor wafer. A concise description of the related art of sputtering devices can be found in U.S. Pat. No. 4,198,283, issued to Class, et al., on Apr. 15, 1980, entitled, xe2x80x9cMagnetron Sputtering Target and Cathode Assembly.xe2x80x9d Typically, sputtering equipment includes a vacuum chamber, a target containing the material to be sputtered, a process gas source that provides a process gas to the vacuum chamber, and equipment to generate an electric field. The target forms part of a cathode assembly in the evacuated chamber containing the process gas, which is typically an inert gas, such as argon. The electric field is applied between the cathode assembly and an anode in the chamber, and the gas is ionized by collision with electrons ejected from the surface of the cathode, i.e., the electric field generates a plasma between the target and the susceptor, and accelerates the ionized gas atoms towards the target. The positive gas ions are attracted to the cathode surface, where they impact the target and dislodge particles from the target material. Once free from the target, these dislodged particles deposit themselves upon the substrate as a thin film.
One method of enhancing conventional sputtering processes is to arrange magnets behind or near the target to influence the path taken by electrons within the sputtering chamber, thereby increasing the frequency of collisions with sputtering gas atoms or molecules. In this type of magnetron sputtering device, the magnetic field is arranged orthogonal to the generated electric field. By increasing the plasma density proximate the target, the number of impacts on the target increases which directly correlates to an increased rate in film deposition on the substrate.
Sputtering apparatus are particularly sensitive to target shape for a number of reasons. The electric field distribution in the vicinity of the plasma discharge is influenced by the target shape because the target shape imposes a boundary condition upon the electric field in accordance with well-known laws of electrostatic theory. Moreover, the shape changes during the useful life of the target as target material is eroded away. Consequently, optimizing the target shape will enhance uniform deposition on the substrate throughout the useful life of the target.
The magnets are typically nickel, iron, or other ferromagnetic material, often assembled from separate segments to form an annular or rectangular body where the segments are separated by small gaps.
It is generally understood that atoms ejected from the surface of a sputter target leave at a variety of angles and that, at the vacuum levels employed in sputtering systems, the mean-free-path of the ejected metal atoms is small in comparison to the dimensions of the vacuum chamber, so that randomizing can occur. This randomization is due to collisions and gas scattering of sputtered species. In order to preserve the flux, the target-to-wafer distance is kept at a minimum to limit randomization.
In conventional magnetron sputtering, low pressures are implemented such that ejected atoms are not randomized. In ionized pvd, randomization or thermalization is needed to ionize the metal species that are sputtered off the target. By operating at high density plasma, a high ionization is achieved which ultimately may yield an isotropic distribution.
It has been a goal of manufacturers of sputtering systems to provide means for imparting greater directionality to the ejected target atoms which reach the semiconductor wafer.
Other prior art methods to improve directionality have included the following: a) increasing the distance between the sputter source and the substrate, wherein only those atoms that start out traveling at an angle close to an angle normal to the substrate will reach the substrate; b) installing a collimating filter between the source and the substrate, where the filter is essentially a network of elongated cell-like structures, each cell having an axis to the substrate surface such that atoms traveling at an acute angle are intercepted by the cell walls; c) plating using an ion beam; and d) applying an rf bias to the wafer substrate causing a negative charge to build up in a known manner, which in turn, causes gas and metal ions in the chamber to arrive to the substrate at angles close to the wafer normal.
Limitations, however, exist with the above-described methods. Increasing the distance between the sputter source and the substrate generates material loses and lower deposition rate, and is considered very inefficient. Similarly, when a collimating filter is installed, much of the target material is wasted, i.e., it does not get deposited on the substrate, and instead is accumulated on the filter cell walls. Ion plating causes space charge effects that prevent the use of a beam with sufficient flux to provide an acceptable deposition rate. Lastly, applying an rf bias on the wafer may cause undue electrical stress to the wafer elements.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for making a physical vapor deposition source for depositing metalization layers onto a substrate with an improved degree of directionality.
It is another object of the present invention to provide a method for making a directional source for depositing metal layers which have an acceptably high deposition rate.
A further object of the present invention is to provide for uniform erosion/removal of material from a physical vapor deposition sputtering target.
Another object of the present invention is to prevent redeposition of target material back onto the target.
Yet another object of the present invention is to provide a method for shaping a non-planar target as means of controlling target erosion.
Still another object of the present invention is to provide a method for shaping of a non-planar target as means of improving target life and utilization.
A further object of the present invention is to provide a method for shaping of a non-planar target as a means of controlling particulate generation for ultra-clean magnetron sputtering applications.
Yet another object of the present invention is to provide a method for shaping of a non-planar target as a means of controlling process characteristics such as Rs uniformity, step coverage, and the like.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of modeling non-planar sputtering target shapes including the steps of: a) selecting a first non-planar sputtering target geometry; b) dividing the non-planar sputtering target into a finite number of target segments, each segment defining a surface area of the target; c) calculating for each of the target segments a contribution of sputtered material from each of the other of the target segments; and, d) calculating the net erosion for each of the target segments.
This method may further include the steps of: e) selecting a second non-planar sputtering target geometry; f) performing the steps (b) through (d) on the second non-planar sputtering target geometry; and, g) comparing the net erosion calculations for the first and second non-planar target geometries.
Additionally this method may include: h) calculating process parameters for each of the target geometries; and, i) comparing the process parameter calculations for the first and second non-planar target geometries.
Furthermore, one may select a non-planar target geometry wherein the geometry is parabolic, cylindrical, elliptical, trapezoidal, hemispherical, or cone shaped. Other target shapes may include: cylindrical-elliptical, cylindrical-parabolic, cylindrical-trapezoidal, or cylindrical-domed.
A second non-planar target geometry may be selected, wherein the geometry is different from the first non-planar target geometry and is parabolic, cylindrical, elliptical, trapezoidal, hemispherical, or cone shaped. Each non-planar target may have a corrugated top surface.
In a second aspect, the invention relates to a method for optimizing a non-planar sputtering target shape comprising the steps of: a) selecting an initial non-planar sputtering target geometry; b) dividing the non-planar sputtering target into a finite number of target segments; c) calculating for each of the target segments a contribution of sputtered material from each of the other of the target segments; d) calculating the net erosion for each of the target segments; e) calculating the redeposition of sputtered material from each of the target segments; f) calculating the net deposition from each of the target segments on a wafer; g) minimally altering the target geometry and performing steps (b) through (f); and, h) repeating the step (g) until the wafer has a calculated uniform thin-film deposition of target material and optimized minimum amounts of the redeposition and the target erosion.
This method may further including the steps of: i) calculating process parameters for the selected target geometry; and, j) comparing the process parameter calculations during the step (g) such that the process parameters are optimized.
In a third aspect, the invention relates to an apparatus for sputtering particles from a magnetron type target onto a substrate, the apparatus comprising: a) a vacuum chamber for enclosing the target and the substrate; b) a process gas source; c) the magnetron type target having a geometry optimized by the method delineated above such that the target geometry is calculated to control erosion and redeposition of target material; d) a voltage source for producing an incident electric field to accelerate ionized gas atoms towards the target; and, e) a magnetic field source comprising: i) a rotating magnet; ii) downstream electromagnets; and, iii) a main magnet stack.
This apparatus may further comprise a target having a geometry comprising parabolic, cylindrical, elliptical, trapezoidal, hemispherical, or cone shaped targets. In this apparatus, the main magnet stack and the rotating magnets are comprised of electromagnets or permanent magnets.
In a fourth aspect, the present invention relates to an apparatus for sputtering particles from a magnetron type target onto a substrate, the apparatus comprising: a) a vacuum chamber for enclosing the target and the substrate; b) a process gas source; c) a trapezoidal sputtering target; d) a voltage source for producing an incident electric field to accelerate ionized gas atoms towards the target; and, e) a magnetic field source comprising: i) a rotating magnet; ii) downstream electromagnets; and, iii) a main magnet stack; and, f) an electric field induced plasma stream.
In a fifth aspect, the present invention relates to an apparatus for sputtering particles from a magnetron type target onto a substrate, the apparatus comprising: a) a vacuum chamber for enclosing the target and the substrate; b) a process gas source; c) the magnetron type target having a geometry optimized by the method delineated above such that the target geometry is calculated to control erosion and redeposition of target material; d) a voltage source for producing an incident electric field on the target; and, e) a magnetic field source comprising: i) a rotating magnet; ii) downstream electromagnets; and, iii) a main magnet stack.
In a sixth aspect, the present invention relates to a program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform the method steps for modeling non-planar sputtering target shapes, the method steps comprising: a) selecting a first non-planar sputtering target geometry; b) dividing the non-planar sputtering target into a finite number of target segments, each segment defining a surface area of the target; c) calculating for each of the target segments a contribution of sputtered material from each of the other of the target segments; and, d) calculating the net erosion for each of the target segments.
In a seventh aspect, the present invention relates to an apparatus for sputtering particles from a magnetron type target onto a substrate, the apparatus comprising: a) a vacuum chamber for enclosing the target and the substrate; b) a process gas source; c) the magnetron type target having a geometry comprising parabolic, cylindrical, elliptical, trapezoidal, hemispherical, or cone shaped targets; d) a voltage source for producing an incident electric field on the target; and, e) a magnetic field source comprising: i) a rotating magnet; ii) downstream electromagnets; and, iii) a main magnet stack.
In a eighth aspect, the present invention relates to a magnetron type target for physical vapor deposition having a parabolic, cylindrical, elliptical, trapezoidal, hemispherical, or cone shaped geometry.
In a ninth aspect, the present invention relates to a magnetron type target for physical vapor deposition having varying thickness and a parabolic, cylindrical, elliptical, trapezoidal, hemispherical, or cone shaped geometry.